Scientists have recognized that there is a significant particle flux from the surface waters of aquatic environments such as oceans and lakes to the deeper zones and underlying sediments. This phenomenon is conventionally referred to as the vertical particle flux. Quantitative measurement, as well as analysis of the composition of the vertical particle flux, is of significant interest to scientists. Vertical particle flux data provides insights into many important aquatic processes, including the rate and magnitude of downward transport of particulate materials, seasonality of downward fluxes, coupling between vertically stratified ecosystems, and water column regeneration rates. Conventional, pre-sediment trap water sampling techniques could not quantify, and generally did not even detect the vertical particle flux phenomenon. It was not until sediment traps were placed at various depths in the water column that this "rain" of sinking particles became evident.
Sediment traps have been used widely in recent years in an attempt to quantify aquatic particle fluxes. Early sediment traps were essentially containers, such as cans, placed open end up in an aqueous environment to collect particles sinking through the water column. Hydrodynamic disturbances in the open-ended containers caused loss of the collected particulate material from the trap during retrieval of the trap, as well as washout of particulate material from the trap during collection. These sample losses resulted in significant inaccuracies in particle flux measurements. The size and frequency of hydrodynamic disturbances, e.g., eddy formation, and the liquid flow pattern within and around a trap varies substantially depending upon trap geometry, particle type and the local current regime.
In general, it is primarily the passively sinking (inanimate) particle flux that is of interest in vertical flux studies. The organic components of this particle flux are labile toward microbial degradation, requiring that sediment traps used for collection periods of any significant duration (e.g., 2315 in excess of one day) be treated with biocides to prevent decomposition of the collected sample. In open traps, zooplankton and other living aquatic organisms, referred to as "swimmers", enter the sediment collection chamber and distort the vertical particle flux measurements. In some aquatic environments, the portion of the vertical particle flux measurement attributable to swimmers may significantly exceed that portion attributable to passively sinking, inanimate particles. Additionally, "grazing" by swimmers on collected particulate material further distorts passive vertical particle composition and flux measurements. Considerable attention has been devoted to the identification of swimmers in and their removal from sediment samples, as well as accounting for inaccuracies resulting from the presence of swimmers in sediment samples.
Another inadequacy with respect to conventional sediment traps is that a trap having a single collection chamber cannot provide temporal resolution of the vertical particle flux. To quantify the vertical particle flux over time, several traps may be separately deployed and retrieved at various time intervals. This process is time consuming and labor intensive, and therefore highly impractical, particularly in aquatic environments that are inaccessible by virtue of their distance from inhabited regions.
Many attempts have been made to provide sediment collection traps that accurately reflect natural vertical particle fluxes. Loss of collected particulate material resulting from hydrodynamic disturbances has been addressed by placement of baffles at various locations within traps, or utilization of lid mechanisms that allow trap deployment and retrieval in a closed configuration. Sediment trap configurations have included narrow-mouthed, jar-like containers, as well as wide-mouthed funnels having narrower sample chambers. These sediment trap design innovations have been somewhat successful in reducing particle loss due to hydrodynamic disturbances, particularly during retrieval of the traps.
Exclusion of swimmers from the particle collection zone of sediment traps and preservation of the particulate material during collection pose difficult challenges. The dissolved poison or preservative must be isolated from other areas of the trap to prevent the killing of swimmers and consequent distortion of the collected sample composition. Relatively dense brines are typically used to isolate poisons and preservatives in a sediment collection zone, and to retain dissolved or leached sample components in the collection zone. Sediment traps having fine screens positioned within the traps above the collection zone to exclude living organisms from the collection area have been devised. Alternatively, labyrinth-type sediment traps have been developed that utilize a series of funnels to direct passively sinking particles to a collection chamber, while allowing mobile zooplankton to "escape" into a secondary poisoned chamber.
Despite these efforts, the recent U.S. Global Ocean Flux Study Workshop on Sediment Trap Technology and Sampling sponsored by the National Science Foundation concluded that current sediment trap technology is inadequate to exclude living organisms, and that a high priority should be given to eliminating swimmers from sediment trap collections. There was considerable discussion during the workshop regarding the need for more accurate collection techniques and preservation of the composition of collected samples. Additionally, the workshop report noted that hydrodynamic biases must continue to be monitored. U.S. Global Ocean Flux Study, "Sediment Trap Technology and Sampling", Report of the U.S. GOFS Working Group on Sediment Trap Technology and Sampling, published August 1989.
The incentive to design an improved sediment trap came out of recent field studies on the performance of conventional (open) sediment traps that were carried out in cooperation with colleagues Dr. Cindy Lee (Marine Sciences Research Center, State University of New York) and Dr. Stuart Wakeham (Skidaway Institute of Oceanography). The main purpose of the research project was to investigate the ability of commonly used poisons and preservatives to prevent alteration and decomposition of different types of organic matter collected in sediment traps. This was accomplished both by laboratory studies (Lee et al., in press) and by deploying sediment traps treated with different combinations of poisons and preservatives in a local marine embayment, Dabob Bay, and determining the organic compositions and fluxes after collection periods ranging from weeks to months. Although several effective biocides were identified in the field and laboratory (Lee et al., in press), it became clear that conventional (unvalved) sediment traps were nevertheless subject to a variety of washout, reprocessing, and swimmer problems that could only be addressed by an improved physical design (Lee et al., in press; Wakeham et al, in preparation; Hedges et al, in preparation). This effort would not have been initiated without the clear demonstration of artifacts intrinsic to open sediment traps that resulted from the project with Drs. Lee and Wakeham.
Accordingly, it would be advantageous to develop a sediment trap for collecting vertical particle flux samples in aquatic environments that includes one or more of the following features: provides isolation of the collected sediment sample from washout by ambient water currents; prevents live organisms from entering the collection chamber of the trap and reduces sample distortion resulting from grazing by swimmers; reduces measurement biases introduced as a result of hydrodynamic effects; allows temporal resolution of vertical particle flux measurements by providing collection of a plurality of sediment samples during a single deployment period; preserves dissolved as well as solid species within the collection chamber; provides a flexible, modular design and programmable monitoring and control functions; and is capable of operating as a self-contained assembly at depths of up to several thousand meters.